1. Field of the Invention
This invention relates in general to methods and devices for dispensing fuel in gas turbine engines. More particularly, but without limitation thereto, this invention relates to gas turbine fuel nozzles and methods for maintaining the fuel which is conveyed therethrough below the temperature which would cause overheating or coking of the fuel.
2. Description of the Prior Art
Gas turbine fuel nozzles which disperse fuel into the combustion area of turbine engines such as airplane engines are well known. Generally these nozzles are attached to an inner wall of the engine housing and are spaced apart around the periphery of the engine to dispense fuel in a generally cylindrical pattern. For example, 30 nozzles could be spaced about the fuel-dispersing zones of a turbine engine. These turbine engines can be arranged with single annular or dual annular fuel dispensing zones. For the engines with dual annular fuel dispensing zones, the nozzles can have two tips on each nozzle body to allow the nozzle to spray or atomize fuel into each of the annular fuel dispensing zones. Thus, an engine with 30 dual-tip nozzles would have 60 nozzle tips. Valves can regulate flow of fuel to each of the tips. This can vary the flow of fuel to the dual annular fuel dispensing zones.
A particular problem with gas turbine fuel nozzles is that the nozzles must be located in a hot area of the engine. This heat can cause the fuel passing through the nozzle to rise in temperature sufficiently that the fuel can carbonize or coke. Such coking can clog the nozzle and prevent the nozzle from spraying properly. This is especially a problem in nozzle or engine designs which provide for fuel flow variations. In these engine or nozzle designs, the fuel flow through some nozzles is reduced to a low flow condition or a no flow condition in order to more efficiently operate the engine at a lower power. Flow through the other nozzles is maintained at a higher flow during this low or no flow use of some of the nozzles. In dual annular combustors, nozzle tips to which fuel flow starts immediately for starting and other low power operations are often referred to as pilot nozzle tips and nozzle tips to which fuel flows at relatively higher rates at high power conditions are often referred to as main nozzle tips.
In nozzles or nozzle tips with low or no flow conditions, the stagnant fuel can become heated to the point where coking will occur despite the fact that the low or no flow condition does not heat the engine as much as the high flow condition. This is because the stagnant fuel has a sufficiently long residence time in the hot nozzle environment that even the lower heat condition is sufficiently high to coke the fuel.
In nozzles or nozzle tips with high flow, the engine design can be such that the high flow condition produces a very high heat condition around the nozzle. In this situation the fuel flowing in the high flow condition may coke despite its high flow rate because of the very high heat condition produced in the engine surrounding the nozzle. This is especially true near the tip of the-nozzle in nozzles with two or more tips.
One method which has been used to insulate the nozzle and reduce the tendency to coking is to intentionally provide a stagnant fuel insulation zone surrounding the fuel conduit. The stagnant fuel cokes in this insulation zone and this coke then has excellent insulation characteristics to provide insulation to the fuel conduit. However, when there is little or no flow in a nozzle passage or tip, this method offers little or no protection from coking in the fuel passage. The residence time of fuel in the low or no flow condition can be such that all possible insulation techniques are ineffective.